The present invention relates to image signal processing systems for evaluating objects present in the image, and more particularly to such systems which compensate for variation in the image signal due to changes in the object's illumination.
As automated assembly equipment becomes more sophisticated, it is desirable to add the sense of vision to such devices. This would enable the equipment to locate the position of an object being manufactured, as well as to inspect the object for the presence of components or the proper location and size of specific features. To this end, various vision systems have been devised which to generate and analyze a one or two dimensional electrical image of the workpiece, which image conventionally is composed of a series of image elements.
A common image analysis technique distinguishes the workpiece from its background based on the relative brightness of portions of the image. The instantaneous image signal voltage varies with the brightness of the corresponding image elements. By processing only image elements which have a brightness above or below a set threshold level the object can be separated from the background. For example when the object is brighter than the threshold, a simple analysis technique counts image elements having a brightness above the threshold level with the sum representing the width or area of the object.
One problem associated with this technique is that changes in lighting intensity alter the relative image element brightness. For example, as the source of artificial illumination ages, its light output changes. In addition, natural and artificial illumination levels may vary throughout the day. A change in illumination is translated into a change in the brightness of the image and the relationship between the object and background brightness levels and the threshold employed to distinguish between these image regions.
To overcome this problem, compensation systems have been used to measure the change in the image signal and adjust the threshold proportionally. An example of such a system is disclosed in U.S. Pat. No. 4,855,830. In other systems the camera iris and amplifier gain are varied to maintain the brightest and darkest image elements at relatively constant signal levels.
One advantage of machine vision systems is the ability to inspect an object moving rapidly along an assembly line. In such an application, the image sensor may continuously acquire images even when an object is not present in its field of view. When an object is not present in the image, conventional lighting compensation techniques may attempt to adjust the iris and gain of the sensor to place the brightest and darkest image elements at the defined levels, as if an object was present. Thereafter, when an object enters the sensor's field of view, the sensor signal became saturated and several iterations of the compensation technique were required to properly adjust signal levels to analyze the object. The process takes time and requires that the assembly line run at a slower speed to accommodate the vision system. Therefore, it is desirable to provide a lighting compensation technique which will recognize when an object is not present and not overcompensate.